The present invention relates to transient voltage protection means and, more particularly, to a novel current-limited spark gap circuit for providing voltage transient protection to a load.
Many electrical and electronic components are subject to damaging or destructive effects due to high voltage transients. It is well known that a spark gap is an effective way of clamping a voltage transient, although a spark gap may itself be destroyed by the effects of a follow-through current. This follow-through current is generally limited only by the line impedance in series with the spark gap, whereby very large currents can flow in the spark gap arc path initially ionized by the transient. Thus, where a load is connected to the A.C. power line, and a spark gap transient protector is placed in parallel with the load to be protected, the power line inductance, between the A.C. power source and the parallelled spark gap and load, is the only significant factor limiting limits the follow-through current through the ionized spark gap when a transient occurs.
It is well known that transient voltage spikes on the A.C. power line, such as caused by switching of inductive and other loads or by nearby lightning strikes, can disturb or destroy electronic circuitry powered from the A.C. power line. It is perhaps less well known that voltage spikes in the 1-3 kV. range can easily destroy an ordinary incandescent lamp, even if the transient pulse lasts only a few microseconds. In some cases, destruction of the incandescent lamp appears to result from concentration of the transient energy in the cathode-fall of the resulting arc to the lamp filament. There is also evidence that the line-frequency (e.g. 60 Hz. in the United States) follow-through current, in the path initially ionized by the transient arc, is the cause of some failures. This same line-frequency follow-through current has also been shown to destroy spark gaps placed, for transient protection, in parallel with incandescent lamps. Unless the spark gap structure is physically large enough to dissipate the nearly short circuit energy delivered by the A.C. power line, between the time the transient ionizes the gap and the next zero-crossing of the power line current waveform, the spark gap will be destroyed. Typically, a gap structure capable of dissipating the energy introduced into the spark gap by the follow-through current is impossibly large for utilization with the component originally to be protected by the spark gap. It is therefore highly desirable to provide a transient protector using a spark gap through which the follow-through current is limited to a value permitting the spark gap to survive.